The invention relates generally to sealing of arc tubes, and more specifically to arc tubes used in ceramic metal halide (CMH) lamps. This invention relates particularly to a heat shield design and methods used in the sealing process of arc tubes.
Ceramic metal halide lamps are generally comprised of a polycrystalline alumina arc tube containing an ionizable fill and having a pair of main thermionic electrodes at the ends. A typical arc tube configuration comprises a central portion, often referred to as a hollow tube, and two arc tube legs attached to respective ends of the central portion. In most applications the electrodes include a relatively high percentage of tungsten. The electrodes are supported by inleads which typically include a thin niobium wire portion extending hermetically through a glass seal in the end of the lamp.
The sealing of the electrodes is typically done by placing the arc tube in a furnace of very high temperature. Certain CMH arc tube geometries are prone to cracking during the transient thermal process in which the electrode is hermetically sealed into the arc tube leg thus leading to yield loss in the manufacturing process.
Typically the body of the arc tube is made of polycrystalline alumina. When the arc tube is placed in the furnace, the high temperature causes the seal glass to melt and penetrate into the arc tube leg thus sealing the electrodes to the arc tube.
Another problem with the manufacturing process is that the arc tube experiences a large amount of hoop stress. Hoop stress usually refers to the stress that builds up around the circumference of the arc tube due to adverse temperature gradient. As a result, the arc tube begins to crack at the regions where maximum hoop thermal stress is experienced.
Therefore, what is desired is a heat shield structure that can be used in the manufacturing process that reduces the temperature gradient in the critical region of the arc tube and thus prevents cracking.
Briefly, in accordance with one embodiment of the invention, a heat shield structure is provided for use in sealing electrodes of an arc tube. The heat shield structure comprises a plurality of layers of alternating thermally conducting material and thermally non-conducting material. The heat shield prevents the arc tube from cracking when the electrodes are being sealed.
In another embodiment, a method is provided for sealing arc tubes while preventing cracking of the arc tube. The method comprises sealing a pair of electrodes on the arc tube. The sealing process is implemented using a furnace. The thermal gradient generated by the sealing process is reduced by implementing a heat shield in the furnace. The heat shield comprises alternating layers of thermally conducting materials and thermally non-conducting materials.